Lithium secondary batteries are mainly used as power supplies in portable devices and the like. Portable devices and the like have a variety of capabilities and thus consume a large amount of electric power. For this reason, lithium secondary batteries are required to increase battery capacities and to enhance charge-discharge cycle characteristics. High-power, high-capacity secondary batteries for use in electric tools such as an electric drill, hybrid cars and the like are increasingly required. Conventionally, lead-acid secondary batteries, nickel-cadmium secondary batteries and nickel-metal-hydride secondary batteries are mainly used in these fields. Compact, lightweight and high-energy density lithium-ion secondary batteries, however, are highly expected to be used, and thus lithium-ion secondary batteries having excellent large-current load characteristics are desired.
In general, in the lithium secondary batteries, lithium salt such as lithium cobaltate is used as a cathode active material, and carbonaceous material such as graphite is used as an anode active material.
Mesocarbon spherules are widely used as graphite serving as a anode active material. The production process of the mesocarbon spherules is, however, complicated, and this makes it extremely difficult to reduce cost of the mesocarbon spherules.
In Graphite, there are natural graphite and artificial graphite. The natural graphite is available at low cost. However, it is shaped like scales. Thus, when the natural graphite and binder are mixed into a paste and the paste is then applied to a collector, the natural graphite is oriented in only one direction. When charging is performed using such an electrode, the electrode expands in only one direction, and this causes degraded performance of the electrode. Although it is suggested that natural graphite be spherically granulated, the spherically granulated natural graphite collapses to become oriented in the same direction by being pressed when electrodes are produced. In addition, since the surface of the natural graphite is active, a large amount of gas is generated in the first charging, resulting in reduced beginning efficiency and a poor cycle characteristic.
Artificial graphite typified by graphitized products from petroleum oil, coal pitch, coke and the like is available at relatively low cost. It is high in strength and resistant to collapse. However, needle coke that is easily crystallized is likely to form a scale-like shape to become oriented in the same direction. Non-needle coke is likely to form substantially spherical particles, but it often has a slightly low discharge capacity and poor beginning efficiency.
Under these situations, instead of mesocarbon spherules, various inexpensive graphite materials for battery electrodes are being researched. Patent document 1 discloses a carbon material for an anode in a lithium secondary battery, namely, graphitized carbon powder prepared by subjecting carbon powder made of pitch in the presence of a boron compound to heat treatment, characterized in that the coefficient of thermal expansion (CTE) of the carbon powder, the interplanar spacing (d002) of graphite plane as measured by X-ray diffraction, the length (Lc) of a crystallite in the direction of the C-axis and the ratio (R=I1360/I1580) of the strength of 1360 cm−1 band to the strength of 1580 cm−1 band as measured by Raman spectroscopy using an argon laser are CTE≦3.0×10−6° C.−1, d002≦0.337 nm, Lc≧40 nm and R≧0.6, respectively.
Patent document 2 discloses a carbon material for an anode in a lithium secondary battery, namely, graphitized carbon powder obtained by graphitizing green coke powder produced from at least one of coke raw materials of petroleum-derived or coal-derived heavy oils after it is heated and oxidized under an atmosphere of oxidized gas, characterized in that, the interplanar spacing (d002) of graphite planes of the carbon powder as measured by wide angle X-ray diffraction, the length (Lc) of a crystallite in the direction of the C-axis, the coefficient of thermal expansion (CTE) and the ratio (R=I1360/I1580) of the strength of a peak in the vicinity of 1360 cm−1 to the strength of a peak in the vicinity of 1580 cm−1 as measured by Raman spectroscopy using an argon laser are d002≦0.337 nm, Lc≧30 nm, CTE≧3.0×10−6° C.−1 and R≧0.3, respectively.
In patent document 3, the assignee of the present invention discloses a carbon material for a lithium battery, the carbon material being composed of graphite powder that is obtained by pulverizing and graphitizing calcined coke and is characterized in that the specific surface area is no more than 3 m2/g, the aspect ratio is no more than 6 and the tapping bulk density is no less than 0.8 g/cm3.    Patent document 1: JP-A-H08-031422    Patent document 2: JP-A-H10-326611    Patent document 3: WO 00/22687